1. Field of the Invention
This invention relates to low noise amplifiers (LNA), to integrated circuits and receivers and consumer equipment having such amplifiers, and to methods of amplifying signals.
2. Discussion of the Related Art
It is known to use an LNA to amplify input signals coming from an antenna of a radio receiver. The Noise Figure NF of this circuit is an important parameter and needs to be small. The input impedance of the LNA needs to be matched with the impedance of the antenna. The practical implementation of the impedance matching and the low NF leads typically to considerable power consumption. This can be reduced by using coils in the circuit. Circuits using coils have been reported as achieving very small noise levels (e.g. NF<3 dB). However, they also have a number of drawbacks, such as EM coupling and large die size. They consume large areas of integrated circuits or hybrid packages. To obtain an accurate impedance with such coils, very specific CAD tools and/or several test structures are required, delaying the development, and adding to manufacturing costs as there needs typically more than one iteration to tune the circuit. The input impedance is highly sensitive to parasitics and difficult to control.
To avoid such difficulties, it is known to use a common gate or common base structure as shown in FIG. 1. The arrangement can be single sided or differential, the parts needed for the differential arrangement being shown in dotted lines. A first transistor 10 is coupled in series with a current source 20 and a load resistor 30. The base or gate is tied to an ac-ground, the input signal is applied to a point between the current source and the transistor, and the output is taken from a point between the load resistor and the transistor. For the differential arrangement, a corresponding set of components is shown, including second transistor 50, load resistor 60, and current source 70.
For common base or common gate designs generally, the key properties are wide bandwidth, and a matched input impedance. This structure has the advantage that no coils are needed and the input impedance is very well controlled. Disadvantages of this basic common base or common gate structure include the fact that there is a theoretical 3 dB NF limit and the fact that the circuit is power hungry in order to have a certain input impedance.